Rename `widgets` to panels to align with pajamas guidelines

What does this MR do and why?

Follow up MR to !119108 (merged)

This MR renames widgets to panels for the VSD YAML configuration to align with pajamas guidelines

Updates the related specs, docs, constants and examples that describe how to setup a VSD configuration panel. Ensures that we align our terminology and examples with the pajamas guidelines.

Screenshots or screen recordings

How to set up and validate locally

Setup configuration Project

1. On the top bar, select Main menu.
2. Select Groups and find your group.
3. On the left sidebar, select Settings > General.
4. Navigate to Analytics Dashboards, click Expand.
5. Select the Project that you would like to use to store your YAML configuration file
6. Click Save changes

Setup configuration file

1. On the top bar, select Main menu.
2. Select Projects and find your project.
3. Create a .gitlab/analytics/dashboards/value_streams/value_streams.yaml file in the default branch using the configuration example below
4. Navigate to the value stream dashboard for your specified group, the dashboard so follow the config you specified
5. Re-open the config file and rename the panels key to widgets
6. Navigate back to the value stream dashboard, and you will only see the default config, the widgets key is ignored

Configuration example

# title - Change the title of the Value Streams Dashboard. [optional]
title: 'Custom Dashboard title'

# description - Change the description of the Value Streams Dashboard. [optional]
description: 'Custom description'

# panels - List of widgets that contain panel settings.
#   title - Change the title of the panel. [optional]
#   data.namespace - The Group or Project path to use for the chart panel.
panels:
  - title: 'My Custom Project'
    data:
      namespace: group/my-custom-project
  - data:
      namespace: group/another-project
  - title: 'My Custom Group'
    data:
      namespace: group/my-custom-group
  - data:
      namespace: group/another-group

MR acceptance checklist

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• I have evaluated the MR acceptance checklist for this MR.

Related to #385716 (closed)